The present invention relates to a method for managing charge/discharge operation of secondary batteries, especially to non-aqueous electrolyte secondary batteries.
In recent years, with a popularization of cordless appliances such as portable telephones, lap-top personal computers and the like, demands for higher capacity and higher energy density of the secondary batteries as the power sources of these appliances become more clamorous.
Of the various secondary batteries, non-aqueous electrolyte secondary batteries, represented by lithium ion secondary batteries, are attracting a great deal of attention since these are expected for realizing further higher output voltage and higher energy density. In recent years, the lithium ion secondary batteries with a positive electrode including a complex oxide of lithium and a transition metal, and a negative electrode including a carbon material capable of absorbing and desorbing lithium in a reversible manner are put to practical use.
Normally, the non-aqueous electrolyte secondary batteries are charged up to their fully-charged state and discharged to their exhaustively-discharged state. As proposed in Japanese Unexamined Patent Publications Hei 5-111184, Hei 6-325794 and Hei 7-240235, a so-called constant current-constant voltage charging scheme is widely employed in the method for charging the non-aqueous electrolyte secondary batteries. The disclosed system includes charging a battery at a constant current until the battery voltage reaches a predetermined value and charging it at a constant voltage until the battery is brought to a fully-charged state. A method for detecting the fully-charged state is proposed, for instance, in Japanese Unexamined Patent Publications Hei 6-189466, Hei 7-105980 and Hei 7-235332.
Diagrams in FIG. 1 and FIG. 2 show variance in charging current value and that in charged capacity with time in the constant current-constant voltage charging scheme, respectively. They both represent the case of charging a lithium ion secondary battery with a rated capacity of 700 mAh in conformity with the conventional method, i.e. the constant current-constant voltage charging scheme. As illustrated in the diagrams, the battery was charged at a constant current of 500 mA (hour rate of 0.7 C, where C is the ampere hour rating of the battery) until a closed circuit voltage of the battery reaches 4.1 V at first, then charged at a constant voltage of 4.1 V. The charging was completed when the total charging period reached 2 hours.
As shown in FIG. 1, the charging current value decreases gradually with the time during the constant-voltage charge period. In particular, at a highly charged state which exceeds 90% of the rated capacity, the charging current value is very small, and as shown in FIG. 2. the charging rate is very small accordingly. As illustrated, a long time period is required for bringing the secondary battery to the fully-charged state in the constant current-constant voltage charging scheme.
In order to shorten the charging time in such charging scheme, increasing the current value during the constant-current charge period may be considered. However, according to such procedure, the charging time is hardly shortened, because the degree of decreasing in the charging current value during the constant-voltage charge period is large. On the other hand, if the voltage value during the constant-voltage charge period is made large, a decomposition reaction of an electrolyte in the battery is promoted and the cycle life of the battery is extremely shortened.
Accordingly, a rapid charging method capable of charging the non-aqueous electrolyte secondary battery in a short time as in the case of alkaline electrolyte secondary batteries had not hence been established.
The object of the present invention is to solve the above-mentioned problems and to provide a method for managing a charge/discharge operation of a secondary battery capable of charging a secondary battery, in particular, a non-aqueous electrolyte secondary battery, in a short time, as well as of improving its cycle life.